1. Field of the Invention
The invention relates to optical gratings. More specifically, the invention relates to reducing the variance of grating spacing in an optical fiber attached to an optical module.
2. Description of the Related Art
Normal optical fibers are uniform along their lengths so that a slice taken from any one point on the fiber would look like a slice taken from any other part of the fiber, neglecting any tiny imperfections. However, the refractive index of portions of the fiber may vary, and, in fact, it is possible to make the refractive index of the core glass vary periodically along the length of a fiber, rising then falling, then rising again. Portions of fibers having periodically varying refractive indexes selectively scatter light passing through the fiber, and are called fiber gratings.
Fiber gratings may be fabricated using an ultraviolet light incident on the glass core of the fiber. The ultraviolet light creates fiber gratings by breaking atomic bonds in the germania-doped silica glass of the fiber core, for example. Typically, an external ultraviolet laser illuminates the fiber through a thin, flat slab of silica with a pattern of fine parallel troughs etched on its bottom, which is typically referred to as a phase mask. In the regions covered by troughs of the phase mask, the ultraviolet light breaks bonds in the glass, changing its refractive index and forming a grating. These variations in the refractive index of the core scatter light by what is called the Bragg effect. Bragg scattering selectively reflects a narrow range of wavelengths. The Bragg reflection wavelength, also referred to herein as the selected wavelength, is determined based both on the grating spacing and the effective refractive index of the core. Light at the Bragg reflection wavelength is reflected from the Bragg grating. Likewise, wavelengths other than the Bragg reflection wavelength are not reflected in phase, so the scattered light waves do not add constructively. The result is a simple line-reflection filter, which reflects the selected wavelength and transmits other wavelengths. In practice, reflection increases strongly over a range of wavelengths, with peak reflection at the selected wavelength. Fiber Bragg gratings can be made to have peak reflection across a narrow band, with nearly square sides. The rest of the light outside the selected band passes through unaffected.
The variation of the reflectivity with the wavelength depends on the nature of the grating. Fine, thin, evenly spaced lines tend to concentrate reflection at a narrow range of wavelengths. Turning up exposures to make a stronger grating will increase reflectivity and broaden the range of reflected wavelengths. Commercial devices using this design select a range of wavelengths as narrow as a few tenths of a nanometer and ranging up to several nanometers wide. The narrow ranges are well matched to the requirements of wave-length division multiplexing where the ability to select specific wavelengths or where pump and signal wavelengths must be combined or separated is important. Other optical devices can do the same thing, but fiber gratings select a narrow range of wavelengths and fit naturally into fiber-optic systems. The wavelength selected by a fiber Bragg grating is typically tuned by changing the refractive index and/or the grating spacing.
In addition, the refractive index is proportional to the temperature of the grating so that when the grating temperature is increased the refractive index also increases. Temperature change also causes thermal expansion or contraction of the optical fiber core, shrinking or stretching the grating period and, therefore, changing the selected wavelength. Changes in the selected wavelength may have a significant impact on the operations of an optical communication system, especially a system that depends on the selection and/or dropping of specific wavelengths by a fiber grating. Therefore, a fiber grating that is stable over time and environmental conditions is desired.
The present invention addresses the above mentioned problems.
In one embodiment an apparatus comprises an optical fiber, an optical module, and an adhesive agent securing the optical fiber to the optical module. The adhesive agent has a water resistance sufficient to maintain a Bragg scattering wavelength within about 0.1 nm of a starting wavelength when exposed to ambient conditions of 85 degrees C. and 85% relative humidity for at least 1,000 hours.
In another embodiment, an apparatus comprises an optical fiber adhered to an optical module with an adhesive agent, wherein the adhesive agent comprises more than 10 weight percent and less than 80 weight percent filler.
In another embodiment, method of maintaining a Bragg reflection wavelength of an optical fiber comprises adhering the optical fiber to an optical module with an adhesive agent comprising more than 10 weight percent and less than 80 weight percent filler.
In another embodiment, in an optical communication system comprising an optical fiber and an optical module, a method for inhibiting water absorption of an adhesive agent securing the optical fiber to the optical module comprises adding an inorganic filler to the adhesive agent so that the filler comprises 10 to 80 percent by weight of the adhesive agent.
In another embodiment, an apparatus comprises an optical fiber, an optical module comprising an optical fiber receiving portion, and an epoxy comprising talc securing the optical fiber to the optical fiber receiving portion of the optical module.
These and other objects and features of the present invention will become more fully apparent from the following description and appended claims taken in conjunction with the following drawings, where like reference numbers indicate identical or functionally similar elements.